A prior art electromagnetic relay of this type generally has the construction depicted in FIG. 17. Such a relay includes an armature block h comprising a pair of armatures d and e disposed in parallel relationship before and behind projecting ends of a yoke b carrying a coil winding a and a permanent magnet f interposed between said armatures d and e, said armature block being pivotally supported about the center of said armatures d and e by means of a round shaft i and a corresponding hole j.
The construction of the relay in which the rotation or rocking motion of said armature block is effected by means of such a shaft-and-hole pivot has the following disadvantages.
(1) The round shaft i must be exactly located at the point of intersection of the centerline X--X of said armatures d and e with the centerline Y--Y of said yoke b. The rationale of such a construction is that because the armatures d and e are to abut against the projecting ends of the yoke b, if the pivot shaft i is displaced out of said point of intersection, the contact areas of said projecting ends deviate from the specified setting to create a gap at one end whilst adequate abutment is established at the other end, the gap resulting in unstable abutment, and, hence, in beats and poor electrical contact. It requires a high degree of engineering skill to establish a perfect registry of the round shaft with said point of intersection.
(2) There must of course be a certain clearance including a fitting margin between the round shaft i and hole j, but because of this clearance, one of said projecting ends may not be contacted if the other end is successfully brought into abutment, thus resulting in unstable contact actions. Furthermore, because the dynamics of this system is such that the forces of abutment (the forces of attraction) at both ends are concentrated on one side of the round pivot shaft and the combined force is utilized as the contact pressing force, an inadequate force of abutment at one end results in a reduced contact-pressing force which is less than the set value and ultimately causes an inadequacy of power of the electromagnetic relay.
(3) In connection with (2) above, if the clearance between the round pivot shaft and the hole is decreased too much, the frictional resistance is increased so much as to detract from the force of pivotal rotation.
Furthermore, the armature block construction of such prior art electromagnetic relay is generally such that the permanent magnet is disposed centrally in an integral molding of segments which serve as said pair of armatures. However, after the molding operation, there arises a dimensional variation between the armatures due to the influences of heat, etc., thus altering the stroke of the armatures relative to the yoke.
Moreover, the general card construction of such prior art electromagnetic relay is that the card portion thereof which is adapted to compress the contact spring is integrally formed with the armature block. This construction is adopted because, in the ordinary electromagnetic relay of this type, a high contact pressure and a great contact wiping action at contact closure are necessary to ensure a high switching capacity and a high resistance to fusion and, hence, the so-called flexure contact drive system of compressing the contact springs with cards is employed. However, with such cards, if one of the contacts has been fused, the armature block may still rotate, in disregard of this trouble, to close the other contact as well, thus causing a circuit trouble, unless certain additional measures are taken.
There is also known an electromagnetic relay of the type wherein one of the contacts is comprised of a flexure contact with the other contact being a lift-off contact. However, there is the problem of imbalance between the spring loads at contact closure, which leads to a poor agreement of spring load with the force of attraction of the electromagnetic relay. Thus, with such a construction, it has been difficult to ensure a sufficiently high sensitivity.